Induction motor controllers are known which in the presence of varying motor loads act to maintain the phase lag angle .phi. between motor voltage and current at a preset demand or reference value by regulating the voltage applied to the motor (the higher the load the higher the applied voltage required). In case of a three-phase motor with its three main windings connected in a star of delta configuration, regulation is effected either in each supply phase or each motor winding (the latter differing from the former for delta-connected motors). Voltage regulation is generally carried out by gate-controlled switching devices which results in the voltage and current waveforms being discontinuous; under these circumstances the phase lag angle .phi. between current and voltage is generally taken to be the angle of lag between the returns to zero of the current and voltage. Since the phase lag angle .phi. is a measure of the power factor of the motor, controllers acting to maintain a particular phase lag angle .phi. are often termed "power factor controllers". Examples of such controllers are to be found in G.B. Patent Specification No. 1,551,644 (NASA), G.B. No. 2,084,360A (National Research Development Corporation), and G.B. No. 2,084,355 corresponding to U.S. Pat. No. 4,361,792 assigned to Chesebrough-Pond).
As used herein, the term "power factor controller" is to be taken as referring generally to controllers of the type which seek to achieve a given phase lag angle by controlling the voltage applied to a single or polyphase induction motor. Furthermore, the terms "phase lag angle" and "phase lag" are to be understood as meaning the angle by which the current lags behind voltage in each motor winding or phase of the motor supply, this angle being measured, in the case of discontinuous current and voltage waveforms, in terms of the period over which current flows, before reversal, after the immediately preceding voltage zero crossing.
With known power factor controllers, the actual lag angle .phi. is measured either by directly or indirectly timing the delay between voltage and current zeros in the same motor winding or supply phase. This measured lag angle is then compared with the reference lag angle value to produce an error signal on the basis of which the applied voltage is varied (an increase in the applied voltage being effected if the measured lag angle is less than the reference value and vice versa). Voltage regulation is, as already mentioned, generally effected by the controlled firing of gate-controlled switching devices, such as triacs and thyristors, connected into the motor windings or supply lines.
The preset reference value of the phase lag angle .phi. should be chosen to satisfy two conditions, namely to give a high power factor (and therefore high motor efficiency) and to ensure that the motor operates on a part of its characteristic which enables the controller to ascertain unambiguously the sign of any load variation from changes in the measured phase lag about the reference value. This latter condition is normally satisfied by arranging for the motor to operate on that part of its characteristic in which, for a given applied voltage, increases in load decrease the actual phase lag and vice versa.
In many known power factor controllers, the phase lag reference value is manually set by adjustment of, for example, a potentiometer arranged to provide a voltage signal indicative of the desired reference phase lag (see, for instance, FIG. 1 of the afore-mentioned GB specification No. 2084360A where a potentiometer 24 is used to set a reference voltage, termed V.sub.REF, which is representative of the desired phase lag angle). Since the optimum value for the phase lag reference is dependent on the characteristics of the motor to be controlled, the process of setting in the reference value generally involves progressive adjustment during trial running of the motor. This setting in procedure is both time consuming and costly.
Clearly, it would be advantageous if a suitable value of the demand or reference phase lag input to a power factor controller could be automatically set upon controller being connected up to a motor.
One such self-calibrating controller is described in the aforesaid G.B. Specification No. 2,084,355A. With this controller, after the motor to be controlled has run up to speed, the controller first operates in a calibrating mode. In this mode, the controller progressively adjusts the firing of the gate-controlled switching device used to control motor energization, until the delay angle between the motor current falling to zero and the next firing of the switching device is equal to the measured phase lag angle. This angle value is then used as the required reference phase lag for operation of the controller in its normal, run mode. The value of the reference phase lag produced by this method is far from ideal and GB No. 2,084,355A gives an empirical formula for deriving a better reference value. This formula contains various constants, the values of which have been determined by experimentation. The described self-calibration method is, in fact, highly empirical in nature and does not necessarily provide the optimum reference phase lag for any particular motor connected to the controller.